In so-called spectral or photon-counting x-ray imaging, x-ray photons incident onto the radiation detector of the x-ray device can be detected individually and their energies can be determined. For this purpose the radiation detector comprises a direct conversion material, such as, for example, cadmium telluride (CdTe) or cadmium zinc telluride (CZT), which produces a pulse-like current signal when a photon enters the material, where the current pulse corresponds to an amount of charge which is indicative of the photon energy. In order to determine the photon energy, the radiation detector generates an electric pulse signal representing the amount of charge generated by an x-ray photon, and the amplitude of this electric pulse signal is allocated to one of plural predetermined energy ranges, which are usually also referred to as energy bins. During an x-ray scan, the numbers of photons allocated to the energy bin are counted, and the x-ray image is reconstructed on the basis of the count numbers. In so doing, x-ray images may be reconstructed which may comprise one sub-image for each energy range and/or which show a material composition of the object with respect to different materials of interest.
A known problem of the direct conversion radiation detectors used in spectral x-ray imaging is their instability resulting from trapped charges. Such charges modify the electric field within the radiation detector and thereby cause a degradation of the charge collecting properties of the detector. This degradation is usually referred to as polarization. As a result of the polarization effect, the amount of charge produced by the material at a given energy of the x-ray photons may change with time and/or photon flux. This may lead to an incorrect assessment of the photon energies, which may result in artifacts in the reconstructed x-ray images.
US 2014/0140469 discloses a computed tomography (CT) device comprising a primary x-ray source and an x-ray detector which produces a signal current in proportion to the total energy of the photons absorbed by the detector. In this regard, the x-ray detector has an unstable gain and the CT device is configured to perform a gain calibration procedure. For this purpose, the CT device comprises a supplemental x-ray source which is used to illuminate the x-ray detector in several supplemental scans. On the basis of the detector signals measured in the supplemental scans, calibration data are determined which is used to correct the signal of the object scan.